Proxy version

I'd like to bump the version to 0.12.0

This new version adds the facility to configure Geminabox as a RubyGems proxy. That is, in proxy mode, if a client requests a gem that is not stored locally, Geminabox will try to get the gem from RubyGems instead. If successful the gem will be stored locally, so in proxy mode Geminabox also acts as a RubyGems cache.

Proxy mode is switched off as default, and can be switched on by either: 

Setting RUBYGEM_PROXY to true in the environment:

    RUBYGEMS_PROXY=true rackup

Or in config.ru (before the run command), set:

    Geminabox.rubygems_proxy = true

Whilst working on this modification, I also had some problems with the Geminabox name spacing. So I have also refactored the name spacing. The main consequence of this is that the main Sinatra app is now Geminabox::Server.

I've also moved some gem loading from the gemspec to Gemfile. Specifically gems that are only needed in the test environment. This should reduce the geminabox gem's dependencies.

Unless anyone has any problems with this new version, I'll merge this and push a new gem in a few days time.